It is known to increase the pattern-writing speed of an electron beam exposure system (EBES) by varying the writing spot dimensions of the electron beam during the process of scanning the beam over the surface of a resistcoated workpiece. Such a variable-spot scanning technique is described in a commonly assigned copending application, Ser. No. 855,608, filed Nov. 29, 1977. In one specific illustrative mode of operation of the described system, two spaced-apart mask plates in the electron column of the system contain respectively different apertures therethrough. By interposing a high-speed deflector between the mask plates, it is feasible to rapidly deflect the image of the first electron-beam-illuminated aperture thereby to alter the portion of the second aperture that is illuminated by the beam. In turn, the beam propagated through the second aperture is demagnified to form a variable-size writing spot on the surface of a resist-coated workpiece.
In one particular embodiment described in the aforecited copending application, four scan lines at a time are traversed by the beam in a raster mode of operation. At each address position along such a four-line-at-a-time scan, any specified one of sixteen different combinations (each comprising zero through four electron spots) is formed by the mask plate apertures and transmitted to the workpiece surface. In that way, the pattern-writing speed of an EBES system is significantly increased.
Considerable interest exists on the part of workers in the microelectronics field in trying to still further increase the throughput capabilities of an EBES-type machine. In attempting to achieve this goal with the aforedescribed two-mask-plate system, the swath width or the number of scan lines traversed during one scan may be increased. But, as this width is increased, the size and complexity of the aperture configurations in the mask plates tend as a practical matter to become excessive and the design of the illumination system becomes more complicated. Moreover, as this width is increased, the number of address positions over which the image of the first aperture must be deflected to achieve different variable-spot combinations also becomes excessive. In turn, this complicates the deflection system design and entails longer deflection times, which are undesirable because they impose a limitation on the overall operating speed of the system.